The invention relates generally to 2,5-disubstituted fluorene monomers, oligomers and polymers derived from them and optoelectronic devices containing the oligomers and polymers.
Polyfluorene homopolymers and copolymers have emerged as preferred materials for active layers in polymer organic light emitting devices. The term “polyfluorene” commonly refers to polymers formed by linking fluorene monomer units at the 2 and 7-positions of the molecule. This linking pattern provides linear conjugation through the aryl groups, giving rise to polymers with attainable band gaps, good charge transport properties and excellent film forming properties. Moreover, functionalization of fluorene at the 2 and 7-positions is readily accomplished since these positions are the most reactive toward electrophilic reagents. Copolymerization of a 2,7-difunctional fluorene with suitable difunctional arylene monomers provides copolymers with tunable band gaps depending on the structure of the comonomers employed.
The successive accumulation of 2,7-linked fluorene segments derived from aryl coupling reactions produces conjugated arylene chains having singlet (S1) and triplet (T1) energies that vary with oligomer length up to about 7 fluorene units (Table 1). Triplet energy data of the 2,5- and 2,7-fluorene materials is partially based on “Comparison of the chain length dependence of the singlet- and triplet-excited states of oligofluorenes” in Chemical Physics Letters, vol. 411, pg. 273, Jul. 1, 2005. It can be seen that for fluorene-based material, the relative energies of the single and triplet are correlated, so that materials that exhibit higher singlet energies also exhibit higher triplet energies as well.
TABLE 1Singlet (S1) and triplet (T1) energies for 2,7-linked fluorenes as a functionof number of fluorene segments.# of Fluorene segmentsS1 (eV)T1 (eV)12.8523.352.3033.192.2553.052.1873.022.16polymer2.972.11
In an OLED device, electrons and holes injected from the cathode and anode respectively combine in an emissive layer producing singlet and triplet excitons that can decay radiatively producing light or non-radiatively producing heat. For polyfluorenes and other conjugated polymers, light emission from the triplet state is a spin-forbidden process that does not compete well with non-radiative modes of decay, so triplet excitons are not very emissive. In devices based on these materials it is highly desirable to extract light from triplet excitons produced in the active layer, since there are statistically three such excitons produced for every singlet exciton. Transition metal complexes, by virtue of spin-orbit coupling, can radiatively decay with an efficiency that competes with non-radiative pathways. When these complexes are incorporated into polymeric OLED devices it is possible to achieve nearly 100% internal quantum efficiency since both singlet and triplet excitons produced in the device can emit light.
The relatively low triplet energies (Table 1) of 2,7-linked polyfluorene materials limits the transition metal complexes that can be incorporated in these devices to those that emit red or orange light from the triplet state. In order to take advantage of desirable features of polyfluorene polymers and copolymers as layers in OLED devices while expanding the color range of the phosphorescent emitters that can be used with these materials, analogues of 2,7-linked polyfluorenes having higher triplet energies are desired.